1. Field of the Invention
The present invention relates, in general, to a return-to-zero modulator using non-return-to-zero data or a carrier suppressed return-to-zero modulator and, more particularly, to an apparatus and method for stabilizing a bias voltage for an external pulse generating modulator, which performs automatic control to automatically detect an optimal bias voltage for the external pulse generating modulator provided in the return-to-zero modulator or carrier suppressed return-to-zero modulator and to maintain the optimal bias voltage during the operation of the modulator.
2. Description of the Related Art
In a long distance optical transmission system using Wavelength Division Multiplexing (WDM), the modulation of a transmission signal has been performed by a Non-Return-to-Zero (NRZ) modulation method using a Mach-Zehnder type external modulator. The external modulator used in this case exhibits a phenomenon (Direct Current (DC) bias drift) in which a transfer curve moves laterally due to the variation of a temperature, etc. An optical transmission signal is distorted due to the DC bias drift caused by the temperature variation, so that an extinction ratio is deteriorated and unstable power is output, thus deteriorating the performance of the system. Therefore, the technology of automatically correcting a bias voltage is required so as to output a stable signal regardless of the temperature variation. Currently, various methods and apparatuses for correcting bias voltages for modulators based on NRZ modulation have been proposed.
However, recently, as a transmission rate of an optical transmission network increases and an interval between channels narrows, various modulation methods based on Return-to-Zero (RZ) modulation instead of conventional NRZ modulation have been researched to perform the modulation of a transmission signal.
A Return-to-Zero modulator (hereinafter referred to as an “RZ modulator”) using NRZ data shown in FIG. 1 is one of modulators based on RZ modulation, which has been newly researched.
Referring to FIG. 1, the RZ modulator using NRZ data includes a Mach-Zehnder type first external modulator (Mach-Zehnder Interferometer 1: MZI1) 101 for receiving a laser beam from a laser light source to generate pulses, and a second external modulator (Mach-Zehnder Interferometer 2: MZI2) 102 for modulating the pulses output from the first external modulator 101 using a NRZ modulation method.
A bias voltage for the first external modulator 101 is located at the peak point of its transfer curve as indicated by reference numeral 201 in a graph of FIG. 2, so that the laser beam is modulated at a voltage amplitude of 2Vπ in synchronization with signal clock/2. As a result of the modulation, the laser beam is converted into a pulse signal indicated by reference numeral 202 of FIG. 2.
The pulse signal 202 is applied to the second external modulator 102. A bias voltage for the second external modulator 102 is located at the point indicated by reference numeral 203 of FIG. 2, that is, the mid-point of the slope of its transfer curve, so that the second external modulator 102 performs an ON/OFF function of gating a signal on and off in response to a data signal, that is, a NRZ modulating function. As a result of the NRZ modulation, a pulse signal carrying data is generated as indicated by reference numeral 204.
For another modulator based on RZ modulation besides the above modulator, there is a Carrier Suppressed Return-to-Zero modulator (hereinafter referred to as a “CSRZ modulator”), which has a construction similar to that of FIG. 1. However, as shown in FIG. 3, there is a difference between the RZ modulator and the CSRZ modulator in that the bias voltage for the first external modulator 101 is located at the bottom point 301 of its transfer curve. Therefore, the laser beam is modulated at a voltage amplitude of 2Vπ in synchronization with signal clock/2. As a result of the modulation, a pulse signal as indicated by reference numeral 302 is generated and a phase difference of 180 degrees exists between neighboring pulses. At this time, referring to the spectrum of the pulse signal 302, a carrier is suppressed and is not shown.
The pulse signal 302 is input to the second external modulator 102, and then modulated into a pulse signal 303 carrying data using NRZ modulation by the second external modulator 102.
In the modulator of FIG. 1 operated as described above, there is no problem even though the second external modulator 102 employs a conventional bias control scheme without change; however, the first external modulator 101 cannot employ a bias voltage stabilizing method, which was used in the conventional NRZ modulation, without change in accordance with the operating characteristics thereof. Therefore, new bias stabilizing technology is required.